3 Answers
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We bought a (VGA) display extender that takes the video + audio signals over up to 95m of Cat 5. As an added bonus, the receiver end of the kit has 4 outputs that can be used simultaneously so this kind of approach may be viable if the displays all need to be close to each other.

A Google search for "DVI extender 4 port" also brings up a lot of kit.

As explained in this excellent post by Jeff, you'll require another PCI video card for a new monitor. Note that you could get a dual-headed video card, so you'll need only one more card for your extra monitors.